The broad, long-term objective of this project is to understand the barrier properties of a confluent osteoblastic monolayer, which will provide insight into the role of the osteoblastic lining cell system in the physiology and pathophysiology of bone. This layer of cells that covers all resting surfaces in bone is expected to serve a "gatekeeper" role in the development of hydraulic and electric resistance as well as passage of material, e.g., water, nutrients, ions, waste products and growth factors, into and out of bone tissue. It is this gatekeeper role that lends bone the ability to function as an organ within vertebrate systems. The Specific Aims of this project are (1) to study the permeability characteristics of water as well as specific nutrients (glucose), ions (calcium and phosphate) and growth factors (insulin, PTH) in a static environment, allowing for simple diffusion and pressure head differences between the apical and basal surfaces as well as (2) in dynamic environments exposed to cyclic flow regimes on the apical and basal surfaces of the membrane. For each of these aims, the effect of PTH on barrier function will be studied in situ. The research design is to measure baseline values for hydraulic and electrical conductivity as well as concentration gradients of water, nutrient (glucose), ion (Ca and P) and growth factor transport (insulin, and PTH or a fluorescent tagged, inert analog such as dextran) due to diffusion and static pressure across the osteoblast cell membrane. A confluent monolayer of osteoblasts from the paraosteosarcoma cell line UMR-106-01-BSP will be used as a model system. To address the first aim, pressure will be provided through a pressure head on either side of the membrane in an Ussing chamber. To address the second aim, an innovative dual flow chamber system will be built, tested and implemented as a physiological cell culture model. In both cases, electrical resistance and hydraulic conductance will be measured across the membrane, and glucose, calcium, phosphate, PTH and insulin will be measured in the apical and basal media. The proposed project represents an innovative departure from conventional understanding of bone physiology. This study is expected to provide insight into new strategies for treatment of bone diseases such as osteoporosis and osteoarthritis.